Why Is My Fish Tank Green? Causes and Fixes

What green water is

Green water is a bloom of single-celled algae that float in the water, which is why it looks like pea soup and not like a coat on the glass. One plant retailer explains that being free-floating lets it multiply very fast.

We found no university or veterinary source on green water in home aquariums. This page rests on retailers, labeled below, which sell some of the products mentioned. Treat their advice as experience, not research.

Is it dangerous?

Usually not directly. One plant retailer says it is not directly harmful, and that breeders even grow it as a food for fry. The real risks are indirect:

• Less oxygen at night. Algae uses oxygen in the dark, so a severe case with little aeration can stress fish.
• Less light for plants. Plants lower down can suffer.
• You cannot see your fish. Problems are harder to spot.
• A die-off can spike ammonia. This is the risk that matters when you treat it.

What causes it

Both things it needs are easy to oversupply.

Cause	Examples
Too much light	Direct sun, a tank by a window, or the light left on too long
Too many nutrients	Overfeeding, too many fish, too little cleaning, or too much liquid fertilizer

A new tank with too much ammonia can also set it off, one retailer says. Our cycling guide covers that.

How to clear it

Water changes alone will not do it. Two retailers agree on that, because the algae multiplies so quickly. Pick a fix below, and deal with the cause as well, or it can return.

Fix	How it works	Watch for
UV sterilizer	Water passes a UV light, which stops the algae reproducing	Costs money, but one retailer says a small one will do
Blackout	Starves the algae of light	Plants suffer, and dead algae can spike ammonia
Polishing pad or filter floss	A fine filter layer traps algae	The sources disagree, as one says fine floss will not remove it
Less light	Shorter hours or a dimmer	Works slowly, and it is needed either way

One retailer that sells UV sterilizers prefers them to a blackout. Another says they can clear green water in a matter of days. We have not tested either.

How to do a blackout

1. Turn off the lights, and keep the filter and all other equipment running.
2. Cover the whole tank with a dark blanket. One retailer warns that green water survives on very little light, so close every gap.
3. Leave it. One retailer says up to 7 days, and another says some people use 7 to 10.
4. Do a large water change afterward, to remove the dead algae.

Test for ammonia during and after any fix. Both retailers warn that a large die-off can spike ammonia. If it shows, a university guide says to act straight away, and our cycling guide has the steps. If this worries you, one retailer suggests moving fish to a temporary tank during a blackout.

How to stop it coming back

• Reduce the light. One plant retailer recommends 6 to 8 hours at most. A service owner quoted by another retailer treats more than 8 to 12 as a cause, so sources differ.
• Move the tank. Keep it out of direct sun.
• Feed less. Overfeeding feeds the algae through the waste it creates. A university guide calls overfeeding a major cause of high ammonia too.
• Keep up with water changes. One retailer suggests at least 20% a week.
• Add live plants. One retailer says fast growers and floating plants compete for light and nutrients. Browse live plants that suit your tank.
• Test. High nitrate or phosphate can feed it, so test your water and your tap.

If your water looks white or has specks, it is a different problem. See why your tank is cloudy.